The invention relates in general to a method for killing aquatic species and in particular to an environmentally friendly method for killing aquatic vertebrate and invertebrate invasive species.
The introduction of nonindigenous (exotic) species has had dramatic negative effects on marine, estuarine, and freshwater ecosystems in the United States and abroad (Elton, 1958; Mooney and Drake, 1986; Chesapeake Bay Commission, 1995) (Shortened literature citations are given throughout the specification. More complete citations are given at the end of the specification.). Effects include alteration of the structure and dynamics of the ecosystem involved, including extirpation of native species (OTA, 1993). The release of ballast water from ships is a major transport mechanism for nonindigenous aquatic organisms (Carlton, 1985) as recognized by the U.S. National Invasive Species Control Act of 1996 (P.L. 104-332).
Approximately 40,000 major cargo ships operating worldwide (Stewart, 1991) pump ballast water on board to ensure stability and balance. Large vessels can carry in excess of 200,000 m3 of ballast which is released in varying amounts at or when approaching cargo loading ports. In 1991, U.S. waters alone received approximately 57,000,000 metric tons of ballast water from foreign ports (Carlton et al. 1994). Ship surveys have demonstrated that ballast water is in general a non-selective transfer mechanismxe2x80x94many taxa representing planktonic and nectonic organisms capable of passing through coarse ballast water intake screens are common. These include bacteria, larval fish and bloom forming dinoflagellates (Chu et al., 1997; Carton and Geller, 1993; Gail and Halsmann, 1997). The diversity of biota in ballast water is reflected in the examples of shipborne introductions of exotic species in the United States shown in Table 1 (NRC, 1996).
Zebra mussel (Dreissena polymorpha) and Asian clam (Corbicula fluminea) introductions are of particular concern given their ability to (1) rapidly cover and change the physical structure of hard submerged substrates; (2) reduce open phytoplankton biomass and hence change desirable pelagic food webs; and (3) act as major macrofouling species of water intake structures used in municipal, agricultural, industrial, and power station water systems (Morton, 1987; Effler, 1994; O""Neill and MacNeill, 1991; Strayer, 1991; MassIsaac et al., 1991).
With regard to (3) above, the flow of water through the intakes carries with it a continuous source of food and oxygen for the organisms and carries away their wastes while the structures themselves protect the mussels from predation and environmental conditions such as wave activity and scouring by ice. Thus, the presence of the mussels and clams not only leads to reduced water pumping capacity but also can act as a seed source for downstream reaches of the water course involved. Control typically involves manual scraping and use of either thermal treatment or biocides (O""Neill, 1996). The biocides used include chlorine, quaternary and polyquatenary ammonium compounds or aromatic hydrocarbons (Waller et al., 1996).
Asian clams are found in 36 of the contiguous states of the United States as well as in Hawaii. Control of this species has been estimated in 1986 to cost the U.S. power industry over one billion dollars per year. The zebra mussel, introduced into the U.S. in 1986, has spread rapidly throughout the Great Lakes, St. Lawrence River, and waterways associated with the Mississippi River. It is expected that the mussels will, within 20-25 years, infest most areas south of Central Canada and north of the Florida Panhandle from the Pacific Coast to the Atlantic Coast. As the zebra mussel advances, the prognosis for native freshwater bivalve populations is bleak, especially for those populations of species considered threatened and endangeredxe2x80x94zebra mussel densities of up to 400,000/m2 have been reported and are thought to be the primary cause of the decline in unionids in the Great Lakes (MassIsaac et al., 1991).
A projected cost of two billion dollars has been proposed for zebra mussel control over the decade of the 1990""s in the Great Lakes with this figure rising exponentially for North America as the mussels continue to expand their range. Expansion is also expected to dramatically increase the molluscicide load carried by our continental river systems. This is already a concern in the Mississippi River drainage and will no doubt lead to stricter future regulation of molluscicide usage at the local, state, and federal levels. Hence, there is a need for the development of alternative, environmentally neutral technologies to control exotic species already imported as well as to eliminate future ballast introductions of exotic species.
The National Research Council (1996) has identified some methods for shipboard treatment of ballast water. They are:
A. Filtration Systems
These systems cause the physical separation and removal of organisms above a certain size through use of deep media filters, coarse and fine strainers and continuously cleaned microscreens. In some cases, flow-through centrifugation systems are used to separate large particles prior to filtering to reduce filter clogging.
B. Biocides
Oxidizing biocides, such as chlorine, can be added to the ballast water by metering concentrated gas or solid chemicals or they can be generated electrically from sea water. Effective biocide concentrations are typically in the range of 1 to 5 mg/l. Non-oxidizing biocides can also be applied such as glutaraldehyde-based chemicals used in industrial water treatment.
C. Thermal Treatment
Inactivation of organisms in ballast water can be achieved by water heating directly through use of waste heat from ship propulsion systems. Ballast water would need to be heated to temperatures in the range of 35xc2x0 C. to 45xc2x0 C. and maintained there for a set period of time.
Additional options for treatment with possible limited applications include electric pulse and pulse-plasma processes, and ozonation. O""Neil (1996) reviews methods used to control zebra mussel colonization in water conduits including power plant cooling systems. They are:
D. Mechanical Controls
Scraping, xe2x80x9cpiggingxe2x80x9d, high-pressure water jetting and abrasive blast cleaning is used to dislodge mussels and their bysal threads allowing for disposal. A prerequisite for mechanical removal is that the facility can withstand some level of infestation before the impacts of fouling to become too great.
E. Oxygen Deprivation (Hypoxia)
Zebra mussels can be killed by hermetically sealing water lines and allowing oxygen consumption of the fouling organisms to drop dissolved oxygen concentrations below that required to support their metabolic requirements. Oxygen deprivation works best at high water temperatures. Conduits must be taken out of service for treatmentxe2x80x94up to 17 days at cold water acclimation temperatures.
F. Thermal Treatment
The periodic flushing of lines with heated water is most easily accomplished when the facility uses raw water for cooling purposes such as in power plant operations. Here, waste heat is available to offset the high cost of heating water up to the required 31-37xc2x0 C. Thermal tolerance increases with increased acclimation temperature and decreases with increasing shell length.
G. Exposure and Desiccation
When back-up lines are available, raw water lines can be shut down for prolonged periods of time and drained to expose and desiccate attached zebra mussels. Exposure may be done at non-freezing or freezing temperatures.
H. Chemical Control
Chemical control includes use of agents that have a toxic effect on the mussels (e.g., metallic ions, copper sulfate) and compounds that oxidize the mussel""s bodies and also act in a toxic manner (e.g., chlorine, chlorine dioxide, ozone, potassium permanganate, chloramines and bromine). Chemical control strategies may be applied annually, periodically, intermittently, semi-continuously, or continuously. Required exposure periods vary with the agent, and its concentration as summarized by O""Neill (1996). 1. Ultraviolet Irradiation
Adult mussels are more resistant to ultraviolet radiation than are veligers given that their shells are opaque. Therefore, this technique is more appropriate for mussel larvae when present in waters of low turbidity
Some patents that address the control of zebra mussels and other fouling exotics in water conduit and tank systems are:
In U.S. Pat. No. 6,221,262 (2001) MacDonald et.al. disclose a method for treating ship ballast water that reduces the concentration of dissolved oxygen and/or carbon dioxide while also adding an anti-oxidant.
In U.S. Pat. No. 6,183,646 (2001) Williams et.al. disclose a method for preventing biofouling based on application of filtration units, an oxidizing agent such as chlorine, and the addition of copper ions.
In U.S. Pat. No. 6,171,508 (2001) Wilson discloses a method and apparatus for treating ballast water that uses oxygenation and deoxygenation steps. Deoxygenation is achieved in a selected vacuum tank outfitted with agitators.
In U.S. Pat. No. 5,932,112 (1999) Wilson discloses a method and apparatus for killing microorganisms in ship ballast water prior to its discharge into coastal waters. Following an oxygenation step, ballast water is deoxygenated under vacuum and with agitation.
In U.S. Pat. No. 5,578,116 (1996) Chang discloses a method and apparatus for controlling zebra mussels in water conduits. Conduits are adapted to maintain an air space in contact with water flowing in the conduit and a vacuum is applied so as to reduce dissolved oxygen levels below those needed to support respiration requirements.
In U.S. Pat. No. 5,948,279 (1999) Chang and Bartrand disclose a method and apparatus for controlling macrofoulers in on-demand water conduits. A vacuum device is adapted to produce oxygen-depleted water both with and without water moving though the conduit.
In EPO Patent 00482813 (1991) Muia and Donlan disclose a method for controlling zebra mussels that mixes the biocyde dimethyl ammonium halide with water.
In contrast to the above-discussed methods, the present invention includes an apparatus and method that uses an inexpensive and safe waste gas (for example, CO2 or combustion exhaust) to control exotics by exploiting animal sensitivity to gas supersaturation. Therefore, the use of traditional chemicals or biocides, such as chlorine, can be eliminated, along with the risk and negative environmental effects associated with their use. Further, required exposure periods are minimized thereby allowing reduced water conduit down times and more frequent (preventive) treatment. Thermal treatment is effective but has an excessive energy requirement given the high specific heat of water. Mechanical controls are labor intensive and require some degree of fouling. Oxygen deprivation treatments require extended treatment periods and a biochemical oxygen demand that may not meet requirements. Ultraviolet irradiation cannot be used in closed conduits and is not effective when treating waters with high turbidity.
The principle utility of the invention lies with its unique capability to cause mortality, at an accelerated rate, of target aquatic invasive species by exploiting their sensitivity to supersaturated concentrations of gases such as carbon dioxide, air, power plant (coal, oil, gas) exhaust gas or a combination of these gases. The method is economical, environmentally safe and applicable to both freshwater and marine waters. Use of the invention is particularly attractive in controlling major macro fouling species of ship ballast tanks and water intake structures/conduits supporting municipal potable water, agricultural, industrial and power station raw water systems.
The present invention exploits the sensitivity of aquatic species to elevated dissolved gas concentrations established through application of gas absorption equipment. Gas stripping equipment is used to remove elevated dissolved gases from treated water prior to release.
The invention will be better understood, and further objects, features, and advantages thereof will become more apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings.